Japanese Patent Provisional Publication Nos. 8(1996)-50206, 7(1995)-306317 and 9(1997)-104866 propose an optical compensatory sheet comprising an optically anisotropic layer formed from a discotic liquid crystal compound, which can be used to enlarge a viewing angle of a liquid crystal display. Publication Nos. 7(1995)-306317 and 9(1997)-104866 further disclose 2,3,6,7,10,11-hexa{4-(6-acryloyloxyhexyloxy)benzoyloxy}triphenylene as a discotic liquid crystal compound suitable for forming an optically anisotropic layer of optical compensatory sheet.
The retardation (Δnd) of the optical compensatory sheet is designed according to optical characters of the liquid crystal cell to be compensated. The value of retardation (Δnd) is a product of birefringent anisotropy (Δn) and thickness (d) of the optically anisotropic layer. Even if the thickness (d) is small, an optically anisotropic layer having sufficient birefringent anisotropy (Δn) can compensate the liquid crystal cell. However, it is very difficult to prepare the optically anisotropic layer having sufficient birefringent anisotropy (Δn) from known discotic liquid crystal compounds, for example, those disclosed in Japanese Patent Provisional Publication Nos. 7(1995)-306317 and 9(1997)-104866.
Japanese Patent Provisional Publication No. 2001-166147 discloses a discotic liquid crystal compound having large birefringent anisotropy. However, the disclosed compound has a poor optical character on wavelength dispersion. In other words, the compound enlarges the wavelength dispersion. The resultant optical compensatory sheet causes an unfavorable chromaticity change in a displayed color image.
Generally, the character on wavelength dispersion inversely relates to birefringent anisotropy. In other words, the more the birefringent anisotropy is increased, the more the wavelength dispersion is usually impaired. It has been desired to develop a discotic liquid crystal compound having large birefringent anisotropy but giving small wavelength dispersion.
It is known that phases formed by discotic liquid crystal compounds are roughly categorized into three phases, namely, a columnar phase, a discotic nematic (ND) phase, and a chiral discotic nematic phase. In the columnar phase, central cores of the discotic molecules are piled up in the form of columns through intermolecular force. In the discotic nematic phase, the discotic molecules aggregate randomly. W. H. de Jeu (Physical properties of liquid crystalline materials, Gordon and Breach Science Publishers, 1980) reports that the discotic nematic phase is seldom observed while the columnar phase is often observed. In addition, the discotic nematic phase of triphenylene compound has been observed only in the case that the triphenylene compound has benzoyloxy or cinnamoyl substituent groups at 2, 3, 6, 7, 10 and 11-positions.
Journal of Materials Chemistry vol. 12, pp. 553 (2002) discloses a compound having a side chain of vinyl ether. However, the disclosed compound is a rod-like liquid crystal compound. Further, the publication is silent about its application to, for example, optical films.